1. Field of the Invention
The invention relates to the field of film bonding. More particularly, the invention relates to the field of bonding a plurality of polycarbonate films to each other.
2. Background of the Related Art
Alternative methods for bonding polycarbonates to polycarbonates, or to other substrates, are currently used in the photonics industry. A broad range of adhesive products, such as epoxies and silicones, have been developed for the optics industry. Urethanes are also used in the ballistic glass industry for bonding polycarbonate to glass.
An object of the invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described hereinafter.
According to the present invention, film stacks are fabricated using solvent welding techniques. An example of such a stack is a polycarbonate retarder stack. Solvents are disclosed which provide high peel strength, and effectively eliminate the optical interface between the substrates. The result is a monolithic structure that is free from internal reflections.
It is an object of the invention to identify a class of solvents that provides adequate working time. Exemplary solvents effectively weld the films together, providing a monolithic structure, without significantly effecting the optical properties of the retarder.
It is a further object of the invention to identify a fabrication process that uses inexpensive solvents in small quantity, is free of cleaning processes, is free of curing processes, and is free of extensive post processing such as autoclaving to remove bubbles (as required with pressure-sensitive adhesives).
It is a further object of the invention to provide a process that yields a high optical level of performance. Multilayer solvent welded stacks have extremely low specular reflection and haze between substrates, maximizing throughput and minimizing stray light. By contrast, adhesives must have a refractive index that is well matched to the substrate in order to attain an acceptable level of performance.
The inventive process also provides stacks with low wavefront distortion. The lack of adhesive with significant optical thickness between layers helps insure that light passing through the stack has minimal distortion. Provided that the substrate is of sufficient quality, stacks with 10-20 layers are sufficiently flat and rigid that they can be used in a free-standing mode.
It is a further object of the invention to produce retarder stacks with a high degree of durability. With polycarbonates only, rather than alternating layers of retarder and adhesive, the basic adhesive specifications, as well as issues of adhesive compatibility with the substrate, are non-issues. When an adhesive is used, concerns include adhesion, optical clarity (typically due to blue absorption), tendency to yellow with illumination, haze, refractive index, hardness (i.e. stress on film), environmental properties, curing process, and viscosity. Requirements in each of these categories greatly diminish the number of usable products, and in some cases no products satisfy specifications for the finished stack product.
In the invention, the lack of a foreign material between substrates eliminates concerns regarding coadhesion, the difference in coefficient of thermal expansion, and tendency for the adhesive to attack (craze or crack) the substrate. Surface treatments such as adhesion promoter or corona treatment to obtain adequate bond strength are also eliminated.
It is a further object of the invention to provide a film stack having a first film and a second film solvent welded to the first film.
An additional object of the invention is to provide a film welding apparatus for solvent welding a first film to a second film. The apparatus has a movable backing surface adapted to support the first film, a pressing roller adapted to press the second film against the first film, and a solvent dispenser adapted to dispense a solvent between the first and second films.
An additional object of the invention is to provide a method of fabricating a film stack from a first film and a second film. The method includes supporting the first film, pressing the second film against the first film, and solvent welding the second film to the first film with a solvent.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.